Generally, in a control device for a vehicle alternator, an engine control ECU (electronic control unit) monitors a field switch ratio (DF: Duty of Field coil) to perform duty control of ON/OFF of a field current for appropriately controlling a power generation amount of the alternator. A DF signal monitored at this time is a PWM signal indicating power generation rate information of the alternator. The DF signal is generated by a regulator of the alternator. In general, the DF signal directly indicates the ON/OFF condition (energized condition/deenergized condition) of the field switch (power transistor).
However, the ON/OFF control of the power transistor is performed by the regulator based on pulse width modulation in the range of about 50 Hz to 200 Hz. Therefore, stability in the ON ratio in every cycle is not ensured, and the ON ratio is not stable. While the alternator operates, the ON ratio changes in every cycle in various patterns of large and small values regardless of the previous and next values, e.g., from 50%→70%→10%→ . . . .
For a conventional device JP 2001-258295, a technique is disclosed in which an averaging circuit including an average value latch circuit is provided for averaging external input signals to eliminate disturbance so that influence of the input signals having the disturbed frequency due to the disturbance noise can be reduced.
Further, JP 2001-145397A discloses a technique of rapidly correcting the variation in the value of the ratio of the DF signal in a case where the ON/OFF ratio of the applied voltage is searched based on a binary search method (repeating values between large and small values) using deviation between the voltage generated by the alternator and the target value. In this case, in searching the ON/OFF ratio of the applied voltage based on the binary search method, in order to improve the variation in the values of the ratios of the DF signal, an ON ratio of 0% or 100% is continuously outputted before searching for rapid correction. The value of the ratio changes up and down until it reaches the search depth limit.